We directly analyzed the in situ production of ephippia and ephippial eggs by an obligate parthenogen Daphnia pulicaria population in a high-mountain lake by using sediment traps during the ice-free period. We recorded the number of eggs per ephippium collected in the traps and studied the zooplankton and phytoplankton communities as well as the nutrient conditions in the lake. Ephippia production was negatively correlated with the light phase of the photoperiod. Numerous empty ephippia were produced, and their number was negatively correlated with the epibiont burden of the chlorophyte Korshikoviella gracilipes on Daphnia. The number of healthy eggs per ephippium in the sediment traps was related to the edible phytoplankton biomass and/or epibiont burden. The proportion of empty ephippia was negatively related to the epibiont burden. These results suggest that the onset and rate of ephippia production is related to the photoperiod, and that the production of ephippial eggs per ephippium increases with greater food availability (phytoplankton biomass and/or epibiont burden). This is the first published evidence that food availability may favour resting egg reproduction under photoperiod conditions that trigger ephippia production.